III-V semiconductor has been popularly applied in the optoelectronic unit, such as LED and solar cell. III-V semiconductor compound is formed on a substrate by epitaxial growth technology, and the composition ratio of IIIA element to VA element can be adjusted to emit or absorb the light in specific wavelength range to match the function of the optoelectronic unit. As the composition ratio of IIIA element to VA element of the III-V semiconductor compound changes, the lattice constant of the III-V semiconductor compound may change. When the difference of lattice constant between the substrate and the epitaxial layer made of III-V semiconductor is too large, more dislocations are formed near the interface between the substrate and the epitaxial layer, and the light-emitting efficiency or the light-absorbing efficiency is decreased so the optoelectronic unit is degraded. Thus, in order to form the epitaxial stack with fewer dislocations, the substrate and the epitaxial stack are generally lattice-matched.